1. Field of the Invention
The present invention relates generally to voltage indication of an energy-storage capacitor used in low-voltage powered DC-to-DC converter devices. More particularly, the invention relates to a voltage sensing means for indirectly sensing storage capacitor voltage in DC-to-DC converters used in electronic flash devices.
2. Description of the Related Art
Many commercially available electronic flash devices monitor the voltage on a high voltage flash capacitor and inform a camera operator, via a ready lamp or by enabling a shutter release mechanism in an associated camera, when there is sufficient firing voltage for flash exposure. In some electronic flash devices, the flash capacitor voltage is monitored such that when the capacitor voltage reaches a predetermined level, the flash charging circuit is turned off. Traditionally, the full charge voltage on a flash capacitor is high (typically, 200 volts or more). In some electronic flash devices, a high voltage zener diode sensing circuit is employed to sense full charge voltage. Such a high voltage zener diode sensing circuit suffers in that it is expensive and also presents a inherent drainage problem to the flash capacitor. Other flash capacitor voltage sensing means have included resistive divided networks with luminescent devices connected across the flash capacitor. Most notably, this type of voltage sensing means suffers from leakage and drainage problems presented to the flash capacitor.
In U.S. Pat. No. 4,630,916, granted Dec. 23, 1986, a circuit for detecting charged voltage of an electronic flash is disclosed. In the '916 patent, a voltage dividing circuit containing a neon tube and a switching element is connected in parallel with a main capacitor of which a highly charged voltage is applied to a flash lamp for flashing. Only when the switching element is turned on does the voltage dividing circuit produce a divided voltage. The divided voltage is compared with a reference voltage The result of the comparison is used for detecting the charged voltage across the main capacitor.
In U.S. Pat. No. 3,863,128, granted Jan. 28, 1975, a voltage monitoring circuit for monitoring the voltage o a DC to DC converter storage capacitor and for limiting the operation of the DC to DC converter is shown. The voltage monitoring circuit includes a programmable unijunction transistor to compare a voltage to be monitored with a corresponding preset reference voltage. The voltage monitoring circuit of the '128 patent is connected across the storage capacitor.
In U.S. Pat. No. 4,540,265, granted Sept. 10, 1985, an energy-saving electronic flash apparatus includes a status indicator apparatus operable for signaling the readiness of the flash unit for the next flash. Energy-monitoring circuitry causes the status indicator apparatus to signal that the flash unit is sufficiently charged. The energy-monitoring circuitry includes a resistor network and a zener diode connected across the storage capacitor of the flash apparatus.
A disadvantage of the above discussed voltage/energy-monitoring circuitry is that each requires numerous components connected across the storage capacitor, resulting in increased susceptibility to drainage and leakage problems on the storage capacitor. As a result, the voltage on the storage capacitor is undesirably affected. In addition, the components required for directly monitoring voltage across the storage capacitor are high voltage components. Such high voltage components are costly.
In U.S. Pat. No. 4,068,150, granted Jan. 10, 1978, a voltage indication means for an electronic flashing device is shown. The electronic flashing device comprises a DC to DC converter circuit having a main discharging capacitor. The charged voltage of the main discharge capacitor is indicated by utilizing the fact that the charged voltage of the main capacitor is in equivalent relation with a voltage generated in the DC to DC converter. In one embodiment of the '150 patent device, the voltage indication means comprises a resistive network and a luminescent device connected across the primary winding of an oscillating transformer.
A disadvantage of the '150 voltage indication means is that it requires a luminescent device directly in the voltage sensing circuit. As a result, the voltage indication means is susceptible to inefficiencies due to the luminescent device, for example, undesired Current leakage. Furthermore, when sensing primary winding voltage, the voltage indication means of the '150 device does not account for noise induced in the primary winding due to the switching ON and OFF of current flow in the primary winding. As a result, inadvertant firing of the luminescent device may occur, causing false indication of the readiness of the electronic flash device and undesirable operation.
It would thus be desirable to provide an indirect voltage sensing means for sensing voltage on a main storage capacitor of a DC-to-DC converter that is simple, cost effective, and provides a high degree of noise immunity.